Method and tool for evaluating a geological formation

ABSTRACT

A method of evaluating a geological formation ( 20 ) of non-homogeneous porosity and permeability, particularly in exploration for hydrocarbon fluids, comprising the steps of:
         (a) inserting a formation evaluation tool ( 30 ) into a borehole ( 10 ) to a location ( 10   a ) within the geological formation ( 20 );   (b) setting the formation evaluation tool ( 30 ) into position isolating interval(s) of the borehole ( 10 ) with a packer arrangement ( 50 ) forming portion of the formation evaluation tool ( 30 ), for sample evaluation for said interval(s);   (c) extracting representative samples of formation fluid from the location within the geological formation ( 20 ) for evaluation under downhole conditions using sampling means ( 32   a,    52   b,    54   b ) operative to extract formation fluid samples over a range of porosity and permeability encountered within the geological formation ( 20 ); and   (d) analysing the formation fluid samples and measuring formation ( 20 ) permeability.       

     A formation evaluation tool ( 30 ) for practising the method, and including an packer arrangement ( 50 ) for isolating intervals of the borehole ( 10 ) for sample acquisition and evaluation, is also disclosed. The method and tool ( 30 ) are particularly suited to geological formations comprising complex carbonate rocks such as mudstones, Wackestone, Packstone, Boundstone and crystalline carbonate rocks which typically include features such as vugs and fractures.

FIELD OF THE INVENTION

This invention relates to a method and tool for evaluating a geological formation, particularly a formation containing hydrocarbon fluids.

BACKGROUND TO THE INVENTION

Oil and gas exploration involves investigation of geological formations for locating hydrocarbon reserves fit for commercial exploitation. Such geological formations may be termed reservoir formations. Oil and gas explorers are constantly trying to find better, more reliable, faster and cheaper ways to assess the potential of prospective reservoir formations as exploration costs can range into the tens and hundreds of millions of US dollars. This is a significant challenge as reservoir geology may be highly complex.

To assist this process, the Applicant has developed a formation evaluation device or tool suitable for evaluating geological formations containing hydrocarbon reservoirs. Such geological formations tend to be porous and permeable, the pores containing hydrocarbon fluids of interest for commercial exploration. During exploration, wells are drilled into these geological formations and formation evaluation tools can then be lowered into the well to evaluate whether the geological formation indeed contains hydrocarbons and whether these hydrocarbons can be economically extracted. Measurement of formation permeability and investigation of vertical continuity of reservoirs or compartments within compartmentalised reservoirs is important to such evaluation.

A prior formation evaluation tool is described in Australian Patent No. 551888. Following drilling of an uncased well or borehole, the tool is lowered to desired position within the borehole on a wireline, set in position, and a probe in the form of a sample pad is moved into sealing engagement with a side wall of the borehole. A sample is then drawn—from the permeable stratum of the geological formation—through an aperture within the pad into the tool for evaluation, that is, through analysis of the hydrocarbon fluids contained within the formation of interest by a range of sensors or instruments.

Such a formation evaluation tool or device is particularly successful for evaluation of geological formations containing clastic rocks, rocks—such as sandstones—which comprise fragments (clasts) of pre-existing minerals or rocks transported from their places of origin because such rocks tend to have relatively homogeneous porosity and permeability. Porosity and permeability does not typically vary over a wide range in clastic rock formations. This homogeneity allows good sealing of sample pad against borehole wall and accurate analysis of any reserves of hydrocarbon fluids contained within the formation.

Clastic rocks are different from the typically more complex carbonate rocks. Carbonate rocks containing carbonate minerals such as calcite and dolomite, may be classified in a number of ways. Carbonate rocks may be classified as follows:

Grainstone Grain supporting but Jess than 10% clay

Mudstone: Mud supported

Wackestone: More than 10% grains

Packstone Grain supporting

Boundstone Original components bound together

Crystalline Original depositional texture lost

With the exception of the grainstones, carbonate rocks are complex having non-homogeneous or heterogeneous porosity and permeability. Grainstones, with intergranular pores, as found in the major Arabian fields do have relatively homogeneous porosity and permeability and the Applicant's formation evaluation tool is effective. For other carbonate rocks, porosity and permeability are affected by phenomena including deposition, compaction, cementation, dissolution leaching and consequent formation of vugular pores or “vugs” (vugs being of erratic pore size distribution and not necessarily inter-connected), grain leaching and fracturing.

Formation evaluation is affected by such non-homogeneous porosity and permeability with complex carbonate rock types, for example, tending to have one or more of the following characteristics:

-   -   1. Moldic pores where pores have been formed by leaching after         the rocks have been compacted. Leached grains also fall into         this class.     -   2. Vugs, also formed by leaching, are larger but similar to         moldic pores.     -   3. Cementation of grains within the rock destroying the original         porosity.     -   4. Dissolution, usually as a result of percolating waters, can         create pores of variable size and markedly increase the         permeability of a formation.     -   5. Sub-aerial exposure results in severe leaching by rain water         creating large vugs or cavities which are not necessarily         inter-connected. The extreme of this is the Karst terrain.     -   6. Fracture porosity. Carbonates are rigid, especially when well         cemented, and thus readily fracture when folded. Fractures are         generally sub vertical but may be at any angle.     -   7. Chalk has a special porosity as it is composed of the         skeletal remains of billions of sea creatures (micrite) living         in clean ocean water. Such deposits are usually bounded by         marine clays. Depositional porosity is often greater than 60%         and compaction will reduce such porosity often leaving the pore         pressure to support the overburden. Such rocks are thus often         over pressured.

Evaluation of formations containing the complex carbonate rock types is challenged by the typical considerable variation or heterogeneity in rock porosity and permeability caused by the above phenomena. Such variation may occur even over a very short interval, or within a relatively small region, within a geological formation causing, during formation evaluation, risk of over or under-estimating hydrocarbon reserves. For example, in a naturally fractured formation —in which oil or gas is trapped within the fracture—the fracture acts as a conduit generally allowing formation fluids to flow more freely to the borehole potentially causing the volume of hydrocarbons to be under-estimated. In contrast, in a vugular formation, alignment of formation evaluation tool with a vug may, at least initially, result in an over-estimate of the hydrocarbons present since a vug may allow a high initial flow rate of hydrocarbon into the tool—indicating a significant resource—yet the vug may not be inter-connected with other vugs or reservoirs of hydrocarbons, this lack of inter-connection being indicative of a potentially less valuable hydrocarbon resource.

In addition, non-homogeneous and large pore size, fractures and vugs are likely to cause significant difficulties with sealing of conventional sample pads against borehole walls and this affects formation evaluation as well. In fact, sealing may not be possible. For example, a sample pad may have smaller area than the cross-sectional area of a fracture or fissure where it intersects a borehole. Such sample pad designs which depend on direct sealing between sample pad, having a sample port formed in the surface, and borehole wall are not effective for extracting formation fluid samples from formations including fractures and vugs.

SUMMARY OF INVENTION

It is an object of the present invention to provide a method and tool for facilitating evaluation of a geological formation of non-homogeneous porosity and permeability including those containing complex carbonates including features such as vugs and fractures.

With this object in view, the present invention provides a method of evaluating a geological formation of non-homogeneous porosity and permeability comprising the steps of:

(a) inserting a formation evaluation tool into a borehole to a location within the geological formation;

(b) setting the formation evaluation tool into position isolating interval(s) of the borehole with a packer arrangement forming portion of the formation evaluation tool, for sample evaluation for said interval(s);

(c) extracting representative samples of formation fluid from the interval(s) within the geological formation for evaluation under downhole conditions using sampling means for extracting formation fluid samples over a range of porosity and permeability encountered within the geological formation; and

(d) analysing formation fluid samples and determining formation permeability.

In another aspect of the present invention, there is provided a formation evaluation tool to be inserted into a borehole for evaluating a geological formation of non-homogeneous porosity and permeability and comprising a packer arrangement for isolating interval(s) of said borehole for evaluation; and sampling means for extracting formation fluid samples from said interval(s) for evaluation under downhole conditions wherein said sampling means operates to obtain formation fluid samples over a range of porosity and permeability encountered within the geological formation enabling sampling analysis and measurement of permeability.

In still another aspect, the present invention provides a method of evaluating a geological formation of non-homogeneous porosity and permeability comprising the step of analysing data from samples of formation fluids extracted from the geological formation in accordance with steps (a) to (c) of the above described method. Such analysis may be performed remotely from the borehole or by a party independent of the party conducting steps (a) to (c) of the above described method.

The methods and tool are especially suitable for use in exploration for hydrocarbons, such hydrocarbon fluids being the target formation fluids though geological formations may or may not include such hydrocarbon fluids.

The formation evaluation tool is specially configured with a packer arrangement, as described in further detail below, to enable extraction of representative formation fluid samples for analysis from geological formations with non-homogeneous and widely variable porosity and permeability. Complex carbonate rocks, having one or more of characteristics 1 to 7 listed above, are of particular concern.

To this end, the method and tool advantageously use packer arrangements which incorporate sampling means. Such sampling means, while conveniently relying on pressure differential to extract samples of fluids from the formation, encompass sampling devices at least additional to the resilient sampling pads or probes which require a seal to be formed at the borehole wall before samples can be extracted. Indeed, a preferred sampling device comprises sample port(s) set back from a borehole wall.

The packer arrangement, desirably a dual or straddle packer arrangement included within the Applicant's formation evaluation tool, comprises one packer or a plurality of packers spaced a fixed distance apart. This fixed distance is known. Each packer conveniently comprises at least two spaced sealing elements for sealing and isolating interval(s), for sampling, within the borehole, conveniently with the assistance of drilling muds as often used to facilitate drilling operations. These sealing elements may be used to isolate a significantly shorter sampling interval, and sample volume, than used with conventional packer arrangements which typically comprise packers spaced up to hundreds of metres apart requiring long sampling times and real risk of tool loss. Such long sampling times are not economically efficient. Samples can then be extracted from the sampling interval(s) enabling acquisition of data from said samples for formation evaluation. Such data can be used to evaluate vertical continuity of the geological formation.

Conveniently, sample port(s) are arranged for extracting formation fluid samples between the two or more sealing elements of a packer. Each sample port conveniently communicates—through a formation fluid sample passage—with sensors for evaluating the samples under downhole conditions and these sensors being also included within the formation evaluation tool. The sample port(s) desirably do not form part of the sealing elements when in operation.

Such location of sample port(s) between the sealing elements of a packer allows a relatively small sample volume to be evaluated. This small sample volume may be rapidly drawn down for sampling due to the higher pressures achievable in the sample volume as compared with the significantly larger sample volumes required by conventional dual packer arrangements. This provides potential to greatly reduce testing time and exploration costs making straddle packer arrangements convenient for exploration use since even when packers are used in a straddle arrangement the benefits of smaller sample volume, higher pressurisation and more rapid drawn down are achievable. Where the formation evaluation tool comprises a plurality of packers, sampling ports may additionally be located between the packers (allowing sampling irrespective of whether each packer is inflated or deflated.

The proposed packer arrangement conveniently comprises a plurality of packers each with inflatable, including expandable, sealing element(s). In a tubular string form of formation evaluation tool, inflatable packer(s) may be included as, or integrated within, one or more sub(s) of a string configured for exploration, sampling rather than hydrocarbon production. Such packers are expanded in position by inflating them with fluid through control valve(s). When expanded, the packers isolate an interval (sample interval) of the well and samples of fluid may be drawn into the formation evaluation tool. Once a sample is taken, one or more packers may be deflated and the formation evaluation tool can be moved to a new testing position.

Packers to be used in the packer arrangement may comprise a tubular mandrel or sub with a longitudinal axis and a housing slidable relative to the mandrel which allows accommodation of the sealing elements during positioning of the tool, each inflatable sealing element being substantially concentrically disposed about, and sealingly attached to, the tubular mandrel and communicable with a source of pressurised fluid, such as borehole fluid, to inflate each sealing element when required. The sealing elements are sealingly attached to the mandrel and are contained within a housing which slides relative to the mandrel during inflation and deflation. When inflated, the sealing elements extend radially outward toward the borehole wall to achieve sealing. The sample ports are ideally located in a portion of the housing having low or very low expansion ratio relative to expansion ratio of of the sealing elements.

The packer arrangement may conveniently be deployed in a straddle configuration to evaluate formation properties and especially vertical continuity of any reservoirs or reservoir compartments. Such deployment of packers is beneficial as testing either of the dual packers would permit sufficiently rapid acquisition of samples as described above. Further, under certain safe conditions, both packers may acquire pressure build up curves or pressure draw down curves to get shut in pressures (SIP) that may be used directly to measure hydraulic gradients from which fluid density and other fluid properties can be calculated. For example, analysis of pressure build up and draw down curves, and/or measuring the differential pressure between packers spaced a fixed distance apart, allows determination of the density of fluids, such as hydrocarbons, present at a location within the geological formation taking downhole conditions into account. Traditional gradient testing has depended upon cable displacements based upon surface measurements of such displacements with the erroneous assumption that surface displacements are the same as downhole displacements and this false assumption results in errors on the computed hydraulic gradient and inaccurate evaluation of the formation and potentially costly errors in assessing the value of hydrocarbon fluids in the formation. As described above, this is a particularly acute problem in the case of carbonate formations.

The method and tool of the invention allow convenient investigation of vertical continuity and other hydraulic properties within a geological formation. Vertical continuity is particularly important since nearly all reservoirs are compartmentalised, with low permeability structures or strata separating compartments containing hydrocarbon fluids and water, each compartment likely containing fluids with different properties. Such low permeability structures may prevent or restrict vertical flow, causing a barrier to commercial exploitation of a reservoir. Investigation of the vertical continuity of such compartments or reservoirs allows identification of vertical fluid flow barriers and so the likely performance of a reservoir, and its value, to be estimated.

For example, if one packer is drawn down and pressure at the location of the other packer monitored then an understanding of vertical continuity of the formation can be inferred. This allows better evaluation of hydraulic properties of fluids within the geological formation, particularly the permeability of the geological formation which dictates whether hydrocarbons can be economically extracted, and a more useful assessment of hydrocarbon reserves and value. In this way, the formation evaluation tool allows measurement of permeability of the geological formation with reference to conditions downhole and therefore in a more precise manner than done previously.

Identifying the representative nature of the formation fluid samples, once extracted by the sampling means, is an important feature of the method and tool. Identifying sample representativeness is based on monitoring a signal from a sensor arrangement desirably included within the formation evaluation tool The sensor arrangement includes at least one and preferably more of conductivity, resistivity, temperature, flow-rate, pressure and density sensors. Signals from the selected sensors are transmitted in real time to the surface where an operator or control unit determines representativeness of the formation fluid sample. The operator or a control unit may determine the best time and means to obtain a truly representative formation fluid sample. Consistency of sensor signals indicates sample representativeness and achieving such consistency may at least take significant and undesirable exploration time, with consequential costs, in many conventional exploration operations. The signals from the sensors are critical to correctly identifying components and characteristic properties (pressure, temperature, conductivity, density etc) of the formation fluid(s). Such sensors, and any additional sensors, enable evaluation of formation fluid samples under downhole conditions to avoid problems of sample contamination or variation in the properties of formation fluid samples when conveyed to surface where pressure and temperature conditions may be very different, providing erroneous analysis of the formation and hydrocarbon reserves present within the formation.

The method and tool allow for accurate evaluation of geological formations containing hydrocarbon resources even where such formations have a wide range of porosity and permeability such as in the case of carbonate or complex carbonate rock types. At the same time, the flexibility of the sampling means in coping with variation in porosity and permeability allows for faster and less costly exploration, this being a very important advantage in an industry where exploration efforts may cost in the tens or hundreds of millions of US dollars.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the method and tool for evaluating a geological formation are now described with reference to the accompanying drawings in which:

FIG. 1 is a schematic side section view showing a formation evaluation tool located in a borehole in accordance with one embodiment of the method and tool of the present invention;

FIG. 2 is a detail schematic part side section view showing an inflatable packer used in the formation evaluation tool shown in FIG. 1; and

FIG. 3 is a side section view of an inflatable packer as schematically illustrated in FIGS. 1 and 2.

FIG. 4 is a schematic side section view showing a formation evaluation tool located in accordance with a second embodiment of the method and tool of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to FIGS. 1, 2 and 4, during exploration for hydrocarbon resources within a complex carbonate geological formation 20, a borehole 10 is drilled from the surface 65 into the formation 20 and a wire-line pump through formation evaluation device or tool 30—of tubular string form comprising a number of inter-connected subs 31, 32, 34, 52, 54—is inserted, through lowering, into the borehole 10 for evaluating the formation. Wireline(s) (not shown) convey the tool 30 within borehole 10. Wireline(s) also convey electrical signals (data), representative of formation properties, to a facility at the surface 65. The electrical signals may also be sent to and from a control unit (not shown) for formation evaluation tool 30. Such control unit may be geographically remote from the borehole 10. Although borehole 10 is shown extending vertically, this is not intended to be limiting.

Formation evaluation tool 30, more detail of which is to be provided below, is lowered to a range of sampling locations within borehole 10, these locations being selected to allow sampling of formation fluids and accurate evaluation, under downhole conditions, of any hydrocarbon fluids that may be present when tool 30 is set into position. At each location, when tool 30 is set in position, sample pads of the formation evaluation tool 30 are intended to collect pressurized formation fluid samples into the tool 30 for analysis. Ideally, such sample pads form an effective seal with the borehole 10 but this may not always occur in carbonate formations due to the presence of structures such as vugs and fractures. Karst formations, which contain these structures, may be particularly challenging for evaluation.

To demonstrate this, one such sampling location 10 a, as shown in FIG. 1, coincides with a fracture 24 within the formation 20. Such a fracture is common in carbonate rock formations. Carbonates are rigid, especially when well cemented, and so readily fracture when folded. Fractures are generally sub vertical and fracture 24 has inclined orientation. Sample pads of at least some conventional sampling tools, such as that described in Australian Patent No. 551888, are not well adapted to sampling at fractures. This problem is overcome by the formation evaluation tool 30 as described in detail below.

Formation fluid samples from sampling locations, including location 10 a, for formation evaluation tool are subjected to analysis after extraction from the geological formation 20. The formation evaluation tool 30 is specially configured, as described below, to extract formation fluid samples for analysis from complex carbonate rocks as rapidly as possible. The term “formation fluid” is intended to encompass the native fluid of the formation ideally without any contamination by fluids, such as drilling muds, not naturally present in the formation.

Identifying the representative nature of samples of the formation fluid is based on monitoring an array of signals from a sensor arrangement, shown as block S, provided in formation evaluation tool 30. The various sensors include conductivity, resistivity, temperature, flow-rate, pressure and density sensors. Signals from the sensor arrangement, S, are transmitted in real time to the surface 65 where an operator or control unit determines representativeness of the formation fluid sample. The operator or a control unit may determine the best time and means to obtain a truly representative formation fluid sample. Consistency of sensor signals indicates sample representativeness. However, the signals from the sensor arrangement, S, also enable acquisition of data for identifying components and characteristic properties (pressure, temperature, conductivity, density etc) of the formation fluid(s).

It is to be understood that formation evaluation tool 30 is provided with such sensors, and any additional sensors, to enable evaluation of formation fluid samples under downhole conditions. This is done to avoid problems of sample contamination or variation in the properties of formation fluid samples when conveyed to surface 65 where pressure and temperature conditions may be very different, providing erroneous analysis of the formation and hydrocarbon reserves present within the formation.

The formation evaluation tool 30 comprises a number of subs, upper sub 31, intermediate sub 32 and lower sub 34 of which include sensors, S, and/or electronic components needed for effective operation of the tool 30. Intermediate sub 32 also includes a flow port 32 a which provides an additional sampling point to that described below. The subs 31, 32, 34 and other subs (which include packers 52 and 54 described below) are interconnected by mechanical couplings (adapters) in manner known in the drilling art. FIG. 3 shows how upper sub 31 is connected to packer 52. Adapter 31 e of sub 31 slots into complementary adapter 52 e of packer 52 and mechanical coupling, by interference fitting, is then made.

Formation evaluation tool 30 is configured to enable evaluation of formation fluid samples under downhole conditions to avoid problems of sample contamination or variation in the properties of formation fluid samples when conveyed to surface 65 where pressure and temperature conditions (as measured downhole by pressure and temperature sensors of sensor arrangement S) may be very different, providing erroneous analysis of the formation and hydrocarbon reserves present within the formation.

Formation evaluation tool 30 includes a dual packer arrangement 50, comprising two packers 52 and 54, for isolating interval(s) of the borehole 10 for sample extraction and sample analysis for acquisition of data for formation evaluation. It will be understood that a greater number (than two) of packers could be used in packer arrangement 50. Packers 52 and 54 are spaced a fixed distance apart.

Packer(s) 52 and 54 include inflatable sealing elements 52 a and 54 a and form respective subs within the string of formation evaluation tool 30. Each packer 52, 54 comprises a hollow tubular mandrel 58 with a longitudinal axis. The features of each packer 52, 54 are described, using packer 52 for purposes of illustration, with reference to FIGS. 1 to 3 (FIG. 2, a detail illustration, showing packer 52 in uninflated/deflated and non-deployed condition). However packers 52 and 54 may be deployed in uninflated and/or inflated condition depending on the desired formation evaluation strategy. Both packers 52 and 54 are identical in construction. Packer 52 has a housing 52 d slidable relative to mandrel 58 and which can accommodate sealing elements 52 a (and the sealing elements 54 a of packer 54) during positioning of the tool 30 in borehole 10. The housing 52 d is provided with two sealing elements 52 a concentrically disposed about, and sealingly attached to, the tubular mandrel 58. The sealing elements 52 a are spaced apart with sample port 52 b disposed in a portion of the housing 52 d extending over the distance between them. This portion 52 b of housing 52 d has low expansion ratio in comparison to the significantly higher expansion ratio of the sealing elements 52 a. Packer 54 is designed in the same way as packer 52 as shown in FIG. 4.

The sealing elements 52 a and 54 a are conveniently of polymer or polymer-metal composite material, the metal providing reinforcement as the packers 52 and 54 must be effectively deployed under robust downhole conditions. The material is selected for durability and to reduce problems of permeability or deterioration due to exposure to species present in the downhole environment. Sealing elements 52 a are provided with cavities 52 c to allow inflation with a pressurised fluid, these cavities 52 c being of annular shape when packer 52 is inflated.

Inflatable packers 52 and 54 are placed several metres apart, though still integrated within formation evaluation tool 30, and it will be observed that, in sampling location 10 a, packer 52 is inflated and packer 54 is uninflated (see FIG. 1). Packer 52 is located to sample at the location of fracture 24. Packers 52 and 54 are expanded in position by inflating their cavities 52 c with pressurized fluid, here borehole fluid—supplied from the interior 59 of tubular mandrel 58 through ports or control valve(s) (not shown).

During expansion, housing 52 d slides in one direction along mandrel 58. During deflation, housing 52 d slides in the opposite direction along mandrel 58 with spring 57 being a return spring facilitating return while minimizing hysteresis effects.

Formation fluid samples, as several are typically taken to ensure reproducibility and accuracy, are then drawn—due to the pressure differential between the formation evaluation tool 30 (and more particularly formation fluid passage 56 which also extends through the interior 59 of the tubular mandrel 58) and geological formation 20 at the sampling location 10 a—into the formation evaluation tool 30, through sample port 52 b (or sample port 54 b in the case of packer 54), which is effectively positioned for sampling in the sample interval. Formation fluid samples are then conveyed through formation fluid passage 56 for evaluation under downhole conditions, for example using the sensors, S—here conductivity, resistivity and/or density sensors—which are located in upper sub 31 of the formation evaluation tool 30.

A space 53, is disposed between the spaced inflated sealing elements 52 a of packer 52 and borehole wall 10, this space 53 being located proximate sample port 52 b through which formation fluid sample is drawn into sample port 52 b.

Sample port 52 b is set back from borehole wail 10. Space 53 has relatively small volume (certainly in comparison to the volume 60 downhole of the packer 52). This relatively small volume of space 53, the sealing elements 52 a being spaced no more than a few metres apart, allows higher pressurization and a rapid draw down and rapid formation fluid sample acquisition time.

Once a formation fluid sample is extracted and evaluated, packer 52 may be deflated and the formation evaluation tool 30 can then conveniently be moved to a new testing location. Alternatively, it may be seen that a number of evaluation options for dual packer arrangement 50 are available even when formation evaluation tool 30 is maintained at sample location 10 a. For example, packer 54 could also be inflated simultaneously with packer 52 (as shown in FIG. 4 where packer sealing elements are shown as 54 a) or vice versa. Packer 52 could be deflated and packer 54 inflated. Finally, both packers 52 and 54 could be deflated and sampling undertaken, for example using the upper sub 31 provided with a different form of sample pad 31 a. In any event, use of packers 52 and 54 significantly improves sample flow rate over single or dual probe formation evaluation tools.

In addition to the above sampling at the packers 52 and 54, flow port 32 a allows sampling to be conducted between packers 52 and 54.

Both packers 52 and 54 may acquire pressure build up curves to get shut in pressures (SIP) that may be used directly to measure hydraulic gradients that from which properties of the hydrocarbons and other fluids present within geological formation 20. may be calculated. For example, analysis of pressure build up and draw down curves using packers 52 and 54, in a range of combinations (inflated and deflated), and/or measuring—with pressure sensors or transducers forming part of sensor arrangement S—differential pressure between packers 52 and 54 spaced a fixed distance apart, yields the density of the hydrocarbons and other fluids present at a location within the geological formation taking downhole conditions into account. Traditional pressure gradient testing has depended upon cable displacements based upon surface measurements of such displacements with the erroneous assumption that surface 65 displacements are the same as downhole displacements and this false assumption results in errors on the computed hydraulic gradient and inaccurate evaluation of the hydraulic properties of the formation and potentially costly errors in assessing the value of hydrocarbon fluids in the formation 20. As described above, this is a particularly acute problem in the case of carbonate rock formations.

The method and tool 30 allow convenient investigation of vertical continuity of any reservoirs or fluid containing compartments within a compartmentalised reservoir located within geological formation 20. If packer 52 is drawn down and pressure at the location of the other packer 54 monitored then an understanding of vertical continuity of the geological formation 20 can be inferred. This allows better evaluation of the hydraulic properties of the geological formation 20 particularly the permeability (also termed vertical permeability) of the geological formation 20 which dictates whether hydrocarbons can be economically extracted, and a more useful assessment of hydrocarbon reserves and value. In addition, the formation evaluation tool 30 allows measurement of permeability of the geological formation 20 with reference to conditions downhole and therefore in a more precise manner than done previously.

The formation evaluation tool 30 may includes additional sampling devices. Here, upper sub 31 of formation evaluation tool 30 comprises sample pads 31 a which may, in some cases, engage the wall of the drilled borehole 10 with a view to sealing against that wall and extracting fluid samples from an interval, within non-fractured region 22, having pores of variable size distribution. Sample pads 31 a may be extended radially into position when sampling and retracted when not in sampling position.

Modifications and variations to the method and tool for evaluating geological formations containing hydrocarbons in accordance with the present invention may be apparent to those skilled in the art. Such modifications and variations are deemed within the scope of the present invention. 

1-17. (canceled)
 18. A method of evaluating a geological formation of non-homogeneous porosity and permeability comprising the steps of: (a) inserting a formation evaluation tool into a borehole by wireline, after drilling of the borehole, to a location within the geological formation; (b) setting the formation evaluation tool into position isolating interval(s) of the borehole with a packer arrangement forming portion of the formation evaluation tool, for sample evaluation for said interval(s); (c) extracting representative samples of formation fluid from the location within the geological formation for evaluation under downhole conditions using sampling means for extracting formation fluid samples over a range of porosity and permeability encountered within the geological formation; and (d) analysing the formation fluid samples and measuring formation permeability, wherein said packer arrangement is a dual or straddle packer arrangement comprising a plurality of spaced apart packers, each packer comprising sealing elements for isolating sampling interval(s) within said borehole enabling extraction of samples for acquisition of data for formation evaluation through at least one sampling port located between said sealing elements of a packer.
 19. A method of claim 18 wherein said geological formation includes a feature selected from the group consisting of vugs, fractures, variable diameter pores and moldic pores.
 20. A method of claim 18 wherein the sampling port is set back from a wall of the borehole such that a space enabling sample draw down is disposed between said sealing elements isolating a sampling interval.
 21. A method of claim 18 wherein said data is used to evaluate vertical continuity of said geological formation.
 22. A method of claim 18 wherein measuring differential pressure between packers spaced a fixed known distance apart allows determination of the density of fluids present at a location within said geological formation taking downhole conditions into account.
 23. A method of claim 18 comprising the step of identifying the representative nature of a formation fluid sample, once extracted by the sampling means, by monitoring a signal from at least one sensor, and preferably a plurality of sensors, included within the formation evaluation tool.
 24. A method of claim 18, wherein said sealing elements are inflatable.
 25. A formation evaluation tool to be inserted into a borehole by wireline, after drilling of the borehole, for evaluating a geological formation of non-homogeneous porosity and permeability and comprising a packer arrangement for isolating interval(s) of said borehole for evaluation; and sampling means for extracting formation fluid samples from said interval(s) for evaluation samples under downhole conditions wherein said sampling means operates to obtain formation fluid samples over a range of porosity and permeability encountered within the geological formation enabling sample analysis and measurement of permeability, wherein said packer arrangement is a dual or straddle packer arrangement comprising a plurality of spaced apart packers, each packer comprising sealing elements for isolating sampling interval(s) within said borehole enabling extraction of samples for acquisition of data for formation evaluation through at least one sampling port located between said sealing elements of a packer.
 26. A formation evaluation tool of claim 25 wherein the sampling port is set back from a wall of the borehole such that a space enabling sample draw down is disposed between said sealing elements isolating a sampling interval.
 27. A formation evaluation tool of claim 25 comprising a tubular string including a number of tubular subs.
 28. A formation evaluation tool of claim 25, wherein said sealing elements are inflatable.
 29. A formation evaluation tool of claim 25, comprising at least one sensor for producing signals to be monitored to identify that formation fluid samples are representative.
 30. A formation evaluation tool of claim 29 comprising a plurality of sensors including at least two sensors selected from the group consisting of conductivity, resistivity, temperature, flow-rate, pressure and density sensors.
 31. A formation evaluation tool of claim 29 comprising a tubular string including a number of tubular subs wherein a sub of said tubular string accommodates the sensor(s).
 32. A formation evaluation tool of claim 31 wherein each said packer comprises a tubular mandrel with a longitudinal axis and a housing slidable relative to said mandrel which allows accommodation of inflatable sealing elements, each inflatable sealing element being substantially concentrically disposed about, and sealingly attached to, the tubular mandrel and communicable with a source of pressurised fluid to inflate each sealing element such that, when inflated, the sealing elements extend radially outward toward a borehole wall to achieve sealing wherein a sample port is located in a portion of the housing low or very low expansion ratio relative to expansion ratio of the sealing elements.
 33. A method of evaluating a geological formation of non-homogeneous porosity and permeability comprising the step of analysing data from samples of formation fluids extracted from the geological formation according to the following steps: (a) inserting a formation evaluation tool into a borehole by wireline, after drilling of the borehole, to a location within the geological formation; (b) setting the formation evaluation tool into position isolating interval(s) of the borehole with a packer arrangement forming portion of the formation evaluation tool, for sample evaluation for said interval(s); and (c) extracting representative samples of formation fluid from the location within the geological formation for evaluation under downhole conditions using sampling means for extracting formation fluid samples over a range of porosity and permeability encountered within the geological formation, wherein said packer arrangement is a dual or straddle packer arrangement comprising a plurality of spaced apart packers, each packer comprising sealing elements for isolating sampling interval(s) within said borehole enabling extraction of samples for acquisition of data for formation evaluation through at least one sampling port located between said sealing elements of a packer.
 34. A method according to claim 33 wherein the sampling port is set back from a wall of the borehole such that a space enabling sample draw down is disposed between said sealing elements isolating a sampling interval. 